1. Field of the Invention
The invention relates to a process for the wet-chemical treatment of semiconductor surfaces by exposure to aqueous solutions containing hydrofluoric acid, and also relates to solutions for carrying out this treatment.
2. The Prior Art
Many processes in semiconductor technology in which a semiconductor slice surface is first subjected to a wet-chemical treatment include a subsidiary step in which an already present or formed surface oxide layer is completely or partially removed again from the slice surface. In silicon technology, those subsidiary steps are of particular importance in which, for example, in cleaning or photolithographic processes, just as in heat-treatment or gettering processes, the production and subsequent removal of surface oxide layers are standard operations and often occur several times in succession. By way of example, reference may be made to the cleaning process described by W. Kern and D. Puotinen in RCA Review, June 1970, pp. 187-206 (often described as "RCA-cleaning" in technical jargon). A sequence of wet-chemical oxidation and removal steps is described therein which serves to convert the surfaces of silicon slices, after chemo-mechanical polishing, to a state which is suitable for component manufacture and in which they, therefore, not only have an excellent geometrical quality, but also have a high degree of chemical purity and are free of particles.
In virtually all cases, the partial or complete stripping of the oxide layers present in each case is carried out with the aid of hydrofluoric acid, the latter, in general, either reacting with the oxide in aqueous solution in an immersion bath or, alternatively, via the vapor phase. At the same time, experience shows that the hydrofluoric acid solutions used in aqueous form additionally have a markedly better cleaning action compared with those in vapor form in relation to the heavy metals, in particular iron, which contaminate the slice surface.
Treatment with hydrofluoric acid in aqueous form has, however, a serious disadvantage. Specifically, during the treatment, the number of particles on the slice surface, as a rule, increases markedly, with the result that a further treatment step usually has to be carried out in order to reduce the number of particles to a level suitable for the further processing of the slices. In general, the particles are composed of polysilicic acid and organic substances contained in the solution. For example, in the case of silicon slices having a diameter of approximately 10 cm, their increase is quite likely to amount to 5,000 particles, based on particles in the size range from 0.17 to 1 .mu.m, which can be determined particularly well metrologically. The number of particles can be determined, for example, with the aid of scattered light measuring methods. In these, the slice surface is irradiated, for example, with laser light and the intensity of the scattered light emitted from it measured, which intensity can be correlated with the number of particles.
The number of particles is particularly high if the slice is lifted out of a bath containing hydrofluoric acid solution or if the wet slice surface freshly treated with hydrofluoric acid is exposed to the action of oxygen, as is, for example, also the case in hydrofluoric acid baths having vigorous circulation. Disadvantageously, the number of particles increases still further during subsequent oxidative cleaning steps in an alkaline or an acidic aqueous medium.
Hitherto, attempts have been made to counteract this problem by diluting the hydrofluoric acid solution, i.e., by supplying a fairly large amount of ultra-pure water consequently reducing the number of particles present in the unit volume of the aqueous phase containing hydrofluoric acid. The amounts of ultra-pure water used in this method are, however, appreciable. If, on the other hand, the hydrofluoric acid solution is used in very dilute form as in another known procedure, the necessary exposure time is consequently prolonged to such an extent that a rapid and economically acceptable processing is no longer guaranteed. This applies even to process variants in which the solution is circulated in bubble-free form and is subjected to a filtration at the same time.
The surfaces produced in all such processes always still contain, however, nuclei for further particle growth, with the result that, as a rule, a marked increase in the numbers of particles can be detected again on the slice surface, which previously appeared satisfactorily, in subsequent alkaline and acidic oxidative treatment steps.